Of all Tony's bad decisions, going after Elvira was his worst. Tony should have already recognized how shitty she was to Frank. He just assumed he could fix her, I guess. But she was a know nothing, do nothing, drug addicted whore that's only good for looking at. Tony wanted an actual wife, an intelligent partner who could be a mother to children and tend the household. If he'd gone out and found some young, virginal, unmarried woman who want to have a large family and make her husband happy, then Tony would have calmed down, not been depressed delving deeper into alcohol and drug use to cope. He'd have decreased risk to protect his kids. Instead, Elvira is what caused him to spiral into self destruction. Manny didn't help much either.